The release of more than 370 billion tons of carbon (GtC) from buried early biospheres, adding more than one half of the original carbon inventory of the atmosphere (~590 GtC), as well as the depletion of vegetation, have triggered a fundamental shift in the state of the atmosphere [1]. Raising atmospheric CO2 level at a rate of 2 ppm/year, a pace unprecedented in the geological record, with the exception of the effects of CO2 released from craters excavated by large asteroid impacts, the deleterious effects of pollution and deforestation have reached a geological dimension, tracking toward conditions which existed on Earth in the mid-Pliocene, about 2.8 million years ago [2].

Lost all too often in the climate debate is an appreciation of the delicate balance between the physical and chemical state of the atmosphere-ocean-land system and the evolving biosphere, which controls the emergence, survival and demise of species, including humans. By contrast to Venus, with its thick blanket of CO2 and sulphur dioxide greenhouse atmosphere, exerting extreme pressure (90 bars) at the surface, or Mars with its thin (0.01 bar) CO2 atmosphere, the presence in the Earth's atmosphere of trace concentrations of greenhouse gases (CO2, methane, nitric oxides, ozone) modulates surface temperatures in the range of -89 and +57.7 degrees Celsius and a mean of 14 degrees Celsius, allowing the presence of liquid water and thereby of life [3].

Forming a thin breathable veneer only slightly more than one thousand the diameter of Earth, and evolving both gradually as well as through major perturbations with time, the Earth's atmosphere acts as a lungs of the biosphere, allowing an exchange of carbon gases and oxygen with plants and animals, which in turn affect the atmosphere, for example through release of methane and photosynthetic oxygen (Figure 1).

CO2 is 28 times more soluble in water than is oxygen. Above critical threshold CO2 becomes toxic for certain organisms. Marine organisms are more sensitive to changes in CO2 levels than are terrestrial organisms. Excess CO2 reduces the ability of respiratory pigments to oxygenate tissues, and makes body fluids more acidic, thereby hampering the production of carbonate hard parts like shells. Relatively modest but sustained increases in CO2 concentrations hamper the synthesis of proteins, reduce fertilization rates, and produce deformities in calcareous hard parts. The observed pattern of marine extinctions is consistent with hypercapnia (excessive levels of CO2), with related extinction events [4].

When the concentration of CO2 in the atmosphere rises above a critical threshold, climate shifts to a different state. Any significant increase in the level of carbon gases triggers powerful feedbacks. These include ice melt/warm water interaction, decline of ice reflection (albedo) and increase in infrared absorption by exposed water. Further release of CO2 from the oceans and from drying and burning vegetation shifts global climate zones toward the poles, warms the oceans and induces ocean acidification [5].

The essential physics of the infrared absorption/emission resonance of greenhouse molecules is indicated by observations in nature and laboratory studies, as portrayed in the relations between atmospheric CO2 and mean global temperature projections (Figure 2).

During most of Earth history the oxygen-poor composition of the atmosphere resulted in dominance of reduced carbon species in the air and the oceans, including methane and carbon monoxide, allowing mainly algae and bacteria to exist in the oceans (Figure 3). It is commonly held that, about 0.7 billion years ago, in the wake of the Marinoan glaciation (so-called “Snowball Earth”), oxygenation of low-temperature water allowed development of new oxygen-binding proteins and thereby of multicellular animals, followed by development of a rich variety of organisms - the “Cambrian explosion” [6].

The present state of the biosphere, allowing survival of large mammals and of humans on the continents, developed when CO2 levels fell below about 500 ppm some 34 million years ago (end Eocene) [7]. At this stage, as well as following warm periods in the Oligocene (c. 25 million years ago) and mid-Miocene (about 15 million years ago), development of the Antarctic ice sheet led to a fundamental change in the global climate regime. About 2.8 million years ago (mid-Pliocene) the Greenland ice sheet and the Arctic Sea ice began to form, with further decline in global temperatures expressed through glacial-interglacial cycles regulated by orbital forcing (Milankovic cycles), with atmospheric CO2 levels oscillating between 180 and 280 ppm CO2 (Figure 4). These conditions allowed the emergence of humans in Africa and their migration all over the world (Figure 5) [8].

Small human clans post-3 million years-ago responded to changing climates through migration within and out of Africa . Homo sapiens emerged during the glacial period preceding the 124 thousand years-old Emian interglacial, when temperatures rose by about 1 degree C and sea levels by 6-8 meters relative to pre-industrial [5]. The development of agriculture and thereby human civilization had to wait until climate stabilized about 8000 years ago, when large scale irrigation along the great river valleys (the Nile , Euphrates , Hindus and Yellow River ) became possible.

Since the 18 th century mean global temperature has risen by about 0.8 degrees C. Another 0.5 degrees C is masked by industrial-emitted aerosols (SO2), and further rise ensues from current melting of the ice sheets and sea ice. The polar regions, acting as the “thermostats” of the Earth, are the source of the cold air current vortices and the cold ocean currents, such as the Humboldt and California current, which keep the Earth's overall temperature balance, much as the blood stream regulates the body's temperature and the supply of oxygen.

At 4 degrees Celsius advanced to total melting of the Greenland and Antarctic ice sheets leads to sea levels tens of meters higher than at present. Further rise of CO2-e above 500 ppm and mean global temperatures above 4 degrees C can only lead toward greenhouse Earth conditions such as existed during the Cretaceous and early Cainozoic (Figure 4) [5, 7].

A rise of atmospheric CO2 concentration triggers feedback effects due to warming, desiccation and burning of vegetation, releasing more CO2. The onset of methane release from polar bogs and sediments is of major concern. Ice/melt water interaction proceeds as melt water melts more ice, ice loss results in albedo loss and exposed water absorb infrared heat, resulting in an amplified feedback cycle. Because CO2 is cumulative, with atmospheric residence time on the scale of centuries to millennia, stabilization of the climate through small incremental reduction in emission may not be sufficient to avoid runaway climate change and possible tipping points.

Climate change is appropriately described as a global oxygenation event affecting geological carbon deposits as well as the present biosphere. At 2 ppm/year the pace of carbon oxidation exceeds the highest recorded geological rate of 0.4 ppm/year at the Paleocene-Eocene boundary at 55 Ma, when about 2000 GtC were burnt, triggering an extinction of species [7].

Sea level rise constitutes a critical parameter which reflects all other components of climate change. Since the early 20 th century the rate of sea level rise increased from about 1 mm/year to about 3.5 mm/year (1993 – 2009 mean rate 3.2+/-0.4 mm/year), representing a nearly 4 fold increase since the onset of the industrial age (Figure 6).

The Earth poles are warming at rates 3 to 4 times faster than low latitudes (Figure 7) [9]. The most detailed satellite information available shows that ice sheets in Greenland and western Antarctica are shrinking and in some places are already in runaway melt mode [10]. A new study, using 50 million laser readings from a NASA satellite, calculates changes in the height of the vulnerable but massive ice sheets and found them especially worse at their edges, where warmer water eats away from below. In some parts of Antarctica , ice sheets have been losing 30 feet a year in thickness since 2003 [10].

The consequences of open ended rise in atmospheric CO2 are manifest in the geological record (Frontispiece). The world is in a lag period, when increasing atmospheric energy is expressed by intense hurricanes, increased pressure at mid-latitude high pressure zones and shift of climate zones toward the poles. With ensuing desertification of temperate zones, i.e. southern Europe , southern Australia , southern Africa , the desiccated forests become prey to firestorms, such as in Victoria and California .

There is nowhere the 6.5 billion of contemporary humans can go, not even the barren planets into the study of which space agencies have been pouring more funding than governments allocate for environmental mitigation to date. At 460 ppm CO2-equivalent, the climate is tracking close to the upper stability limit of the Antarctic ice sheet, defined at approximately 500 ppm [5,7]. Once transcended, mitigation measures would hardly be able to re-form the cryosphere.

According to Joachim Schellnhuber, Director of the Potsdam Climate Impacts Institute and advisor to the German government:

“ We're simply talking about the very life support system of this planet ”.

Humans can not argue with the physics and chemistry of the atmosphere. What is needed are urgent measures including

Figure 1. The terrestrial atmosphere as “lungs of the biosphere” – an analogy.

Figure 2. The relations between atmospheric CO2-equivalent (including the radiative forcing of methane) and mean global temperature, according to Charney's climate sensitivity parameter (Hansen et al., 2007, 2008) (IPCC-2007). Circles mark new paleoclimate estimates of atmospheric conditions in the mid-Pliocene (2.8 million years ago) and the mid-Miocene (15 million years ago), with implications to current climate trajectories.

Figure 3. An artist's impression of Earth's oceans as they may have appeared up to about 1 billion years ago, when the oceans were populated by single celled algae and bacteria.